To easily and properly perform handling and management of an upper cover, and closure of a vessel body by the upper cover, in a pressure vessel.

A support mechanism 3 includes: a rotary arm 30 of which the free end part 30a is connected to the upper cover 1, and of which the base part 30b is combined to be axially movable with respect to a vertical rotary central shaft 32; and an urging means 31 interposed between the base part 30b of the rotary arm 30 and a support body 33 of the rotary central shaft 32 to make an upward urging force act on the rotary arm 30. A clamp mechanism 4 includes a pair of right and left clamp members 40, 40. Each of the clamp members 40, 40 has a tightening groove 40a housing an edge 12 of the upper cover 1 and an opening end 20a of the vessel body 2 from the side at the same time, and one end side thereof is rotatably combined with the support body 33 by a vertical shaft 40b. The clamp mechanism 4 includes a connecting means 41 which can tighten to make the other ends of the clamp members 40, 40 be close to each other, and connect them.